Fig. 8. Structure-function analysis of nonpeptide thrombopoietin mimetics. Various pyrazol-4-yli-denehydrazines were prepared as indicated (X) and tested for their efficacy vs recombinant throm-bopoietin in several assays. The luciferase assay measures the extent of luciferase expression in a TPO-responsive cell line in which luciferase expression is driven by a STAT-responsive promoter. The proliferation assay measures the extent of growth of a TPO-dependent cell line (UT7/TPO). The differentiation assay measures the amount of CD41 cell expression when human bone marrow CD34+ cells are grown for 10 d. (Reproduced with permission from ref. 79.)

The recombinant TPOs (rHuTPO and PEG-rHuMGDF) have undergone extensive testing in animal models of overexpression, chemotherapy, radiation therapy, radiopro-tection, bone marrow transplantation, stem cell expansion, HIV infection, and surgery. With only one exception, the results of these animal models have predicted the outcome of similar studies in humans. Only the enhanced white and red blood cell recovery seen in murine chemotherapy studies has never been seen in similar human chemotherapy trials.

Overexpression of TPO has been studied in three models. Mice in which TPO was overexpressed after transplantation of bone marrow cells transfected with MGDF had a more rapid platelet reconstitution than did control mice. Platelet counts increased four-to eightfold and remained increased. Mice had increased numbers of bone marrow and spleen megakaryocytes but ultimately developed marrow fibrosis, extramedullary hematopoiesis, hepatosplenomegaly, osteosclerosis, and anemia (83,84).

In a second model, murine bone marrow cells were transfected to a much higher degree with TPO cDNA and transplanted into mice. Whereas amounts of circulating